Metal oxidation is a problem in numerous industries. For example, acid mine drainage (AMD) is a severe environmental problem that adversely affects many streams in the United States. Acid mine drainage results from the oxidation of pyrite (FeS2) and other metal sulfides such as HgS. As metal sulfides oxidize, the aqueous environment becomes acidified and rich in a variety of metals including iron, manganese, lead, mercury, and others. Generally, AMD is associated with abandoned mines which fill with water and promote oxidation of metal sulfides therein. In addition to abandoned mines, AMD also occurs in areas surrounding refuse piles of coal which accumulate during the coal cleaning process. Extensive leaching can result from these coal refuse piles, particularly during heavy rainfall or snowfall. There is accordingly a need in the art for methods and compositions for prevention of solid dissolution (leaching) resulting from the above-described processes.
Numerous attempts have been made to address the issue of AMD. For example, it is known to provide on-site treatment plants which treat effluent waters with high doses of lime to precipitate heavy metals. However, use of lime has several drawbacks, including the need for high dosages and the production of secondary wastes such as metal hydroxides and gypsum, which must be ultimately sent to landfills.
It is also known to coat pyritic surfaces using microencapsulation technology to retard pyrite oxidation (Vandiviere, M. M, Evangelou, V. P.
1998. Comparative testing between conventional and microencapsulation approaches to controlling pyrite oxidation. J. Geo. ExpL. 64, 161-176; Belzile, N., Maki S., Chen Y., Goldsack D. 1997. Inhibition of pyrite oxidation by surface treatment. Sci. Tot. Env., 196, 177-186; Evangelou V. P. 2001. Pyrite microencapsulation technologies: Principles and Potential Field Application. Eco. Eng. 17, 165-178; all incorporated herein by reference). This technology emphasizes the use of rock phosphate, phosphate, potassium hydrogen phosphate, or silica-based compounds to create a ferric phosphate or ferric silica complex around the pyrite that prevents pyrite oxidation from either oxygen or iron (III). Similar studies have been performed using other phosphate minerals such as hydroxyapatite and fluoroapatite to complex and precipitate Fe(II). For phosphate coatings to be effective, it is necessary to use an oxidizing precursor such as peroxide prior to introduction of a phosphate salt. In the case of phosphate mineral usage, major drawbacks include the fact that as the phosphate complexes form at the surface of the coal it renders the phosphate source inactive, thereby shortening the overall effectiveness of the procedure. Additionally, it is necessary to maintain the pH above 4 for optimal performance. Use of phosphate or rock phosphate to control pyrite oxidation ultimately results in the liberation of large amounts of sulfur which, under reducing conditions, may lead to formation of sulfuric acid. In the presence of an oxygen source, sulfate may form.
Accordingly, there is a need in the art for suitable methods for preventing dissolution and oxidation of metal-containing solids such as pyrite (coal) and metal or metal-coated surfaces. Advantageously, the method should allow binding of metals in the form of a solid-state lattice, rather than requiring binding of free metals, and form stable complexes which remain stable over a range of environmental conditions and over extended periods of time.